Image processing apparatus

ABSTRACT

An image processing apparatus comprises an input device for inputting an image data, a first converter circuit for converting the input image signal to an analog image output and a second converter circuit for converting the input image data to a digital image output. The first converter circuit controls the analog image output in accordance with the output of the second converter circuit. The second converter circuit produces different digital signals in accordance with the input image data and has a threshold matrix to be compared with the input image data.

This application is a continuation of application Ser. No. 07/833,649,filed Feb. 5, 1992, now abandoned, which was a continuation of Ser. No.07/152,024 filed Feb. 3, 1988, now abandoned, which was a division ofSer. No. 06/881,492 filed Jul. 1, 1986, now U.S. Pat. No. 4,783,837,which was a continuation of Ser. No. 06/480,823 filed Mar. 31, 1983, nowabandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image processing apparatus forrecording and displaying an image, and more particularly to improvementin the resolution and tonality of a half-toner image recorded ordisplayed by a laser beam printer or an ink jet printer.

2. Description of the Prior Art

Many methods to produce a half-tone image in a digital printer have beenproposed. Examples thereof are a dither method and a density patternmethod. These methods have been used in many fields for the reasonsthat:

(1) The half-tone image can be displayed by a digital display device,

(2) a hardware configuration of the apparatus is easy, and

(3) satisfactory image quality is attained.

Specifically, as shown in FIGS. 1A and 1B, each of picture cells 8 of aninput image is compared with a corresponding one of elements of athreshold matrix 5 to determine if it is white or black by thresholdcomparison in order to selectively display dots on a display screen 6.

FIG. 1A illustrate the dither method in which each of the picture cells8 of the input image corresponds to one element of the threshold matrix5. FIG. 1B illustrates the density pattern method in which each of thepicture cells 8 of the input image corresponds to all elements of thethreshold matrix 5. Thus, in the density pattern method, each of thepicture cells of the input image is displayed by a plurality of cells onthe display screen 6.

A difference between the dither method and the density pattern methodresides in that one picture cell of the input image corresponds to oneelement of the threshold matrix in the former method while itcorresponds to all elements of the threshold matrix in the lattermethod, and it is not an essential difference. An intermediate methodhas also been proposed, in which one picture cell of the input imagecorresponds to a certain number (for example, 2×2=4 in FIG. 1B) ofelements of the threshold matrix.

Accordingly, there is no essential difference between the densitypattern method and the dither method, the density pattern method and theintermediate method are hereinafter collectively referred to as thedither method. In such a dither method, the threshold matrix can beprepared in many methods. However, a method or apparatus for allowing ahigh quality of image output has not been proposed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an image processingapparatus which produces a high quality image output.

It is another object of the present invention to provide an imageprocessing apparatus which enables recording or display of a stablehalf-tone image with high resolution and high tonality.

It is another object of the present invention to provide an imageprocessing apparatus which reproduces a high quality half-tone imagewith a simple construction.

It is yet another object of the present invention to provide an imageprocessing apparatus which can readily set any desired screen angle.

It is still another object of the present invention to provide an imageprocessing apparatus which causes output dots to form a uniform latticespace.

It is a still another object of the present invention to provide animage processing apparatus which can produce a variable magnificationhalf-tone image with a simple construction.

It is a further object of the present invention to provide an imageprocessing apparatus which produces a color image free from moirestripes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate a dither method and a density pattern method,

FIG. 2 shows a schematic view of a color image recording apparatus towhich the present invention is applicable,

FIG. 3 shows a schematic perspective view of a scanning optical system,

FIG. 4 shows a block diagram of a signal processing system of thepresent invention,

FIG. 5 shows a schematic perspective view of an input device,

FIG. 6 shows a CCD line sensor,

FIG. 7 shows another input device,

FIG. 8 illustrates masking process,

FIG. 9 shows a blackening circuit,

FIG. 10 shows a block diagram of a binarizing circuit and a ternarizingcircuit,

FIGS. 11A-11D show threshold matrices,

FIG. 12 illustrates binary and ternary outputs,

FIG. 13 shows a timing chart for signals in FIG. 10,

FIGS. 14A and 14B illustrate basic cells,

FIG. 15 shows a density pattern of basic cells,

FIG. 16 shows a pattern in which basic cells are connected by density(1) in FIG. 15,

FIG. 17 shows a pattern produced when blacking dots are randomlyproduced,

FIG. 18 shows a pattern in which dots are uniformly arranged by the useof the threshold matrices 1 (B) and (D),

FIG. 19 illustrates a γ-conversion,

FIG. 20 shows a matrix construction for a screen angle of 0 degrees,

FIGS. 21A and 21B show an 8×8 threshold matrix used for black,

FIGS. 22A and 22B show examples of black record dots,

FIGS. 23A and 23B show basic cells for yellow,

FIG. 24 shows superposition of dots of different colors having screenangles,

FIG. 25 shows an image reproduced with different screen angles forrespective colors,

FIGS. 26A and 26B show basic cells in other embodiments,

FIGS. 27A-27C show dot patterns in other embodiments, and

FIGS. 28A, 28B, 29A and 29B show threshold matrices for producing thedot patterns shown in FIGS. 27A and 27B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows a schematic view of a color image recording apparatus towhich the present invention is applicable. In the color image recordingapparatus of FIG. 2, color image information is produced by anelectronic copying machine (laser beam printer) having a plurality ofphotosensitive drums, and images of different colors produced by theelectronic copying machine are recorded in superposition with differentcolors sequentially.

In FIG. 2, numerals 1a-1d denote scanning optical systems. Desired imageinformation is read from an image memory, not shown, and it is convertedto light beams (laser beams) by the scanning optical systems and thelight beams are focused onto photosensitive drums 2a-2d which correspondto cyan (C), magenta (M), yellow (Y) and black (Bl). Developing units3a-3d are arranged close to the photosensitive drums 2a-2d, and chargers4a-4d are arranged to face the photosensitive drums 2a-2d on a side of aconveyer belt 7 which conveys a record paper, not shown. In operation,the modulated light beams from the scanning optical systems 1a-1d arefocused onto the photosensitive drums 2a-2d and the focused images areconverted to electrostatic latent images by a subsequentelectrophotographic process. The electrostatic latent images for therespective colors are developed by the developing units 3a-3d and thedeveloped images are sequentially transferred to the record papercarried on the conveyer belt 7 of the chargers 4a-4d so that a colorimage is reproduced.

FIG. 3 shows a schematic perspective view of one of the four scanningoptical systems 1a-1d shown in FIG. 2. The light beam modulated by asemiconductor laser 11 is collimated by a collimating lens 10 anddeflected by a rotating polygon mirror 12. The deflected light beam isfocused onto the photosensitive drum 3 by a focusing lens 13 called a fθlens and the light beam is scanned in H direction (main scan direction).In the light beam scan, the light beam at the beginning of one-line scanis reflected by a mirror 14 and directed to a detector 15. A detectionsignal from the detector 15 is used as a synchronizing signal in thescan direction H (horizontal direction). This signal is hereinafterreferred to as a BD signal or a horizontal synchronizing signal.

FIG. 4 shows a block diagram of a signal processing system of thepresent invention.

Color image signals for blue (B), green (G) and red (R) produced by aninput device 20 are digitized by eight bits (256 levels), respectively.The input device 20 is shown in FIG. 5. A light is applied to a colororiginal 30 from a light source 37, and a reflected light is transmittedto a CCD line sensor 32 through a mirror 36 and a lens 31. Thus, theimage of the color original 30 is formed on the CCD line sensor 32 andread out with high resolution.

As shown in FIG. 6, the CCD line sensor 32 has 2048 sensors 33 arrangedin three rows, and blue (B), green (G) and red (R) stripe filters 34B,34G and 34R are bonded to the respective rows. The 8-bit picture celldata produced by the input device 20 represents a simultaneous threecolor decomposition of the image data at one point on the original.

FIG. 7 shows another embodiment of the input device 20 used in thepresent invention. Dichroic filters 35a and 35b for three-colordecomposition are arranged behind a lens 31 to decompose the light intothree color components, and the images of the respective colors aredirected to CCD line sensors 32a, 32b and 32c. With the device of FIG.7, three-color decomposed image information for one point on theoriginal is produced as a time-serial signal.

Returning to FIG. 4, the blue (B), green (G) and red (R) 8-bit digitalsignals produced by the input device 20 are processed by a maskingcircuit. Each of the 8-bit image signals from the input device 20 isdivided into a high order four-bit group and a low order four-bit group,and only the high order four bits are masked by the masking circuit 21and the low order four bits are then combined with the high order fourbits to produce an 8-bit data 22.

FIG. 8 illustrates the masking process. The high order four bits of theblue (B), green (G) and red (R) image signals are supplied to a ROM 40as input address information. Assuming that the blue (B), green (G) andred (R) data are represented by hexadecimal numbers

B=9

G=A

R=E

BGR=9AE is regarded as one address and the information at that addressof the ROM 40 is read out. Accordingly, the ROM 40 has 4×3=12-bitaddress. The output information from the ROM 40 also has 12 bits, fourbits of which represent yellow (Y) data, four bits of which representmagenta (M) data and four bits of which represent cyan (C) data.Assuming that the output data for the input address of 9AE is 357, dataof

Y=3

C=7

are produced. The ROM 40 stores all possible values for blue (B), green(G) and red (R).

Since four bits are allocated to each of blue (B), green (G) and red(R), 12-bit data are stored in 12-bit address area (2¹² ≈4K). Thus, thememory capacity may be small. In this manner, high fidelity colorreproduction is attained by the masking process with small memorycapacity. The data conversion (content of ROM 40) by the masking processis experimentally determined in accordance with a chrominancecharacteristic of the recording apparatus and a chrominancecharacteristic of the input device. The masked yellow (Y), magenta (M)and cyan (C) four-bit data are combined with the separated low orderfour-bit data to reproduce the 8-bit image data. That is, thenon-processed blue (B), green (G) and red (R) low order four-bit dataare added to the masked yellow (Y), magneta (M) and cyan (C) four-bitdata.

The above masking process is a kind of blocking process for colorconversion. The masking for only the high order four bits of the 8-bitimage signal means the color conversion in 16 levels, and the additionof the low order four bits means subdivision of the respective leveldata into 16 levels.

Thus, the masked data can be further subdivided and the tonality of theimage is improved.

A blackening circuit 23 of FIG. 4 is now explained with reference toFIG. 9. The masked 8-bit image data is processed by the blackeningcircuit 23 of FIG. 9. (The low order four-bits are converted to the Y,M, and C data.) The blackening circuit 23 converts the masked yellow(Y), magenta (M) and cyan (C) 8-bit data in the following manner.

Y←Y-α·min (Y, M, C)

M←M-α·min (Y, M, C)

C←C-α·min (Y, M, C)

B1←min (Y, M, C)

It compares the yellow (Y), magenta (M) and cyan (C) 8-bit input datafor each picture cell to determine a minimum value

min (Y, M, C)

and sets a block (B1) level to the minimum value, and substracts α times(0<α≦1) of the black (Bl) level from the yellow (Y) , magenta (M) andcyan (C) values. The coefficient α is experimentally determined. The loworder four-bits for the respecitve colors produced by the input devicemay be inverted by hardware inverters to convert them to complementarycolor data.

FIG. 10 shows a detail of a binarizer/ternarizer circuit 24 of FIG. 4.For the sake of simplification, the circuit for only one color isexplained.

An image data 41 for one color (8-bits) of yellow (Y), magenta (M), cyan(C) and black (Bl) is supplied to a comparator 42, a comparator 44a forbinarization (white and black) and a comparator 44b for ternarization(white and gray). Those comparators may be constructed by 8-bittransistor-transistor logic circuits TTL's) such as SN74LS684. Data(threshold values) of the threshold matrix are stored. in a ROM 45a anda ROM 45b. Those ROM's are hereinafter referred to as the binarizing ROM45a and the ternarizing ROM 45b.

The data of the binarizing ROM 45a and the ternarizing ROM 45b are readout in synchronism with the counting by decimal counters 49 and 50. Thedecimal counters 49 and 50 count a picture cell clock 46 and a BD signal48, respectively, and sequentially access horizontal addresses andvertical addresses of the threshold matrices through address lines 47aand 47b, respectively, to read out the data. The threshold matrices areof 10×10 construction as shown in FIGS. 11A-11D. A horizontal (mainscan) direction of the threshold matrix is represented by H-directionand a vertical (sub-scan) direction is represented by V-direction. Thedecimal counter 49 determines the H-direction address of the thresholdmatrix in synchronism with the picture cell clock 46. The decimalcounter 50 determines the V-direction address of the threshold matrix insynchronism with the BD signal 48. The decimal counters 49 and 50 eachmay be constructed by a single conventional TTL such as SN74190.

The binarizing ROM 45a contains two types of threshold matrices as shownin FIGS. 11A and 11B. Similarly, the ternarizing ROM 45d contains twotypes of threshold matrices as shown in FIGS. 11C and 11D. Thebinarizing ROM 45a determines white and black levels and the ternarizingROM 45b determines white and gray levels. The numerals in the thresholdmatrices represent the threshold levels in decimal numbers. One of thethreshold matrices (A) and (B) and one of the threshold matrices (C) and(D) is selected by the input image data 41.

The comparator 42 of FIG. 10 compares the input image data 41 with apreset data 43, and if the input image data 41≧the preset data 43, itsupplies a "1", output to the binarizing ROM 45a and the ternarizing ROM45b so that the input data 41 is compared with the matrices (A) and (C).

If the input image data 41<the preset data 43, the comparator 42produces a "0" output and the input image data 41 is compared with thematrices (B) and (D). Assuming that the preset data is "4", the values0-3 of the input image data 41 are compared with the matrices (B) and(D), and the values equal to or larger than four are compared with thematrices (A) and (C). This can be attained by imparting the output ofthe comparator 42 of FIG. 10 as the high order addresses of the ROM 45aand the ROM 45b. Since each of the outputs of the decimal counters 49and 50 has four bits, light bits (bits 0-7) are used to scan theH-direction and V-direction addresses of the matrices. Thus, one bit isadded to the high order address bit to select one of the matrices (A)and (B) and one of the matrices (C) and (D). Thus, the bits 0-7 of theROM address are used for address specification and the bit 8 is used formatrix selection.

Thus, since the threshold matrices for comparison are selected inaccordance with the density level of the input image data, a highquality output image can be produced.

If the density level of the input image data is low, the unevenness ofdots is obstructive unless the output image dot pattern is uniform. Inthe present embodiment, since the matrices (B) and (D) are selected whenthe density level is low, a uniform dot pattern is produced.

FIG. 12 illustrates binary and ternary outputs. Numeral (2) in FIG. 12denotes an output of one picture cell width, FIG. 12A shows a distanceof movement of a recording light spot, FIG. 12B shows a width of amodulating pulse for the laser beam and FIG. 12C shows a resulting lightintensity distribution. The ternary output in the present invention isproduced by pulse width modulation of one-half picture cell width of thelaser beam shown by (1) in FIG. 12. As seen from FIG. 12C, the ternaryoutput of the one-half picture cell width is intensity-modulatedresulting in variation of peak intensity in accordance with a light spotdiameter. As a result, the ternary output provides an intermediatedensity (gray level). By way of example, in the binary output, the lightspot diameter is approximately 50 μm at a 1/e² intensity point of thepeak intensity and the light spot diameter in the ternary output is 25μm.

The ternary output by the pulse width has the following advantages.

1) Intensity of the laser beam emitted may be fixed.

2) A stable peak intensity is attained by the stable pulse width.

3) Pulse width can be readily modulated.

The hardware configuration for the ternary output is shown in FIG. 10.The output from the comparator 44b for the ternary output is ANDed withthe picture cell clock 46 by an AND circuit 51 and the output of the ANDcircuit 51 is ORed with the output of the comparator 44a for the binaryoutput by an OR circuit 52. Accordingly, when the binary and ternaryoutputs are simultaneously produced, the binary output is selected. Theternary output (gray) is selected only when the binary output is at alow level and the ternary output is at a high level.

FIG. 13 shows a timing chart therefor. The ternary output S2 is producedfrom the comparator 44b in synchronism with the picture cell clock 46shown by S1. The output S2 and the picture cell clock 46 (S1) are ANDedby the AND circuit to produce the ternary output signal S3 of theone-half picture cell width. If the binary signal shown by S4 isproduced, the ternary output and the binary output are ORed to producethe final output signal S5.

In this manner, smooth tonality is attained by the pulse width modulatedternary output.

While the binarization and the ternarization have been explained in thepresent embodiment, an image of excellent tonality can be reproduced byternarization or multi-value processing.

The present invention is not limited to the pulse width modulator sinceother analog half-tone reproduction method such as beam intensitymodulation may be used to reproduce the half-tone image.

The contents of the threshold matrices of FIG. 11 are next explained.

FIG. 14A shows an aggregation of basic cells of the threshold matrix.The aggregation of five cross-shaped basic cells forms one unit. Eachbasic cell comprises 20 elements as shown in FIG. 14B. By sequentiallyblacking the elements of the basic cell the density is represented asshown in FIG. 15. The term basic cell herein used means a pattern ofthreshold values. The threshold values may differ from basic cell tobasic cell.

The five basic cells can be regarded as the threshold matrices of FIG.11 or 10×10 square matrices by moving in parallel selected ones of thebasic cells. Thus, the aggregation of the basic cells of FIG. 14A can beregarded as the 10×10 threshold matrix.

In FIG. 14A, lines connecting corresponding ones of the elements of thefive basic cells 1-5 are inclined lines and angle of inclination is 26.6degrees. In FIG. 16, the basic cells are represented by the density ofFIG. 15 (1) and connected together.

The angle of inclination constitutes a screen angle to prevent moirewhen the color image is outputted. By repetitively connecting the 10×10threshold matrices (aggregations of the basic cells), a continuousscreen angle is attained. The 10×10 threshold matrix has 100 elements(thresholds) and 101 different dots 0-100 by binary data and 202different dots by ternary data can be produced. In FIG. 15, the blackarea of the basic cell is gradually increased. This method is called aFatting method. The threshold matrix of FIG. 11 is blackened by theFatting method in the following manner when the threshold is equal to orhigher than 10. The basic cells 1 and 4 produce the gray level (ternary)and black level (binary) outputs. The basic cells 2 and 3 produce theternary output and the binary output, respectively. Then, the basic cell5 produces the binary output.

The reason for adopting the above method are:

1. When the basic cells 1-5 are constructed as shown in FIG. 15 with thesame threshold, the number of tones is up to 20. By imparting differentthresholds to the basic cells 1-5, the tonality is enhanced. A unit ofthe resolution when viewed as grid points is one basic cell but a unitof the tonality comprises five basic cells (approximately 100 tones).

2. When the basic cells are sequentially blackened, one element at atime, in the order of 1, 4, 2, 3 and 5, the added blackened cell appearsat a course pitch and it is obstructive. By blackening the elements inthree groups, the basic cells 1 and 4, the basic cells 2 and 3 and thebasic cell 5, the pitch of the grid points is reduced to one half andthe dots are not obstructive.

3. Because the ternary output is used, the gradient of the blackening ofthe basic cells is gentle.

When the density data corresponding to the threshold 4 is supplied,uniform dot patterns can be produced with ternary output. For thethresholds 5-9, the basic cells are blackened one element at a time. Inorder to enhance the tonality to a relatively bright portion of theimage, the blackening is effected finely.

The reason for grouping the threshold matrices into (A) and (B), and (C)and (D) is explained below. When only the matrix (A) or (C) is used, theblack dots for the first several tones appear randomly as shown in FIG.17. FIG. 17 shows a pattern when one dot of each of the basic cells 1and 4 of the aggregation of basic cells of FIG. 14A is blackened. Thedot arrangement is random until all of the basic cells 1-5 have beenblackened. When such a pattern is developed by the electrophotographing,nonuniformity of density occurs at an area in which dot pitch spatiallychanges and the tonality is distorted. When an image of a low inputdensity level is to be reproduced by an ink jet printer, thenonuniformity of the recorded dot array is obstructive. Accordingly, itis desirable to form the dots at a uniform density. To this end, use ofthe matrix (A) or (C) only is not sufficient.

By the reason described above, the matrix (B) or (D) is provided.

FIGS. 11B and 11D show the threshold matrices which are used for thefirst several tones. The thresholds 1, 2 and 3 of FIGS. 11B and 11D arearranged to construct the dots at a uniform density. The data of thedensity 3, for example, is compared with the preset data 43 by thecomparator 42 as shown in FIG. 10. Assuming that the preset data is "4",the data of the density 3 is compared with the matrices (B) and (D).FIG. 18 shows an output pattern of the density 3 when the matrices (B)and (D) are selected. It differs from FIG. 17 in that the dots areuniformly arranged. By appropriately setting the data 43, the matrices(B) and (D) are selected so that the dots are arranged at the uniformdensity even if the density level of the image is low.

In this manner, the random arrangement of the dots which appears in thefirst several tones can be eliminated by switching the matrices.

The data of the threshold matrix of FIG. 11 is up to 100. In FIG. 11A,if the image is of very high density, the matrices include five 100 dataand five dots are blackened. This is done by the same reason as that foreliminating the nonuniformity of the dot density in the beginning stage.That is, nonuniformity of white area (called white dots) surrounded byblack dots is prevented, and reduction of the area of the white dots orreduction of density change due to protrusion of a large recording spotwhen one dot is blackened is prevented.

In this manner, approximately 100 tones (levels 0-100) of dotconfiguration are attained.

The yellow (Y), magenta (M), cyan (C) and black (Bl) input data to thebinarizer circuit 24 of FIG. 24 are 8-bit data, respectively, and have256 tones, respectively. FIG. 19 illustrates a γ-conversion method forconverting the 256 levels of image input to 100 levels.

In FIG. 19, the abscissa represents the contents of the elements of thethreshold matrices shown in FIG. 11 and the ordinate represents valueswhich can be represented by the 8-bit image data (that is, 00-FF inhexadecimal number or 256 levels). By determining an appropriate curve60 for executing the γ-conversion (depending on a particular deviceused), the relation between the image data and the threshold level isdetermined. It is necessary to update the data of the respectiveelements of the threshold matrices of FIG. 11 in accordance with thecurve 60 of FIG. 19. By the γ-conversion appropriate values are set tothe elements (thresholds) of the threshold matrices.

The screen angle for preventing moire for the respective colors is nowexplained. As described above, by constructing the threshold matrix, asshown in FIG. 11, the screen angle is 26.6 degrees. This is done onlyfor one color (for example, magenta).

The threshold matrix for cyan (C) is constructed by rotating thethreshold matrix of FIG. 11 by 90 degrees. That is, the H-direction andthe V-direction of the threshold matrix are exchanged. As a result, thescreen angle of 26.6 degrees for magenta is changed to the screen angleof 63.4 degrees. The threshold matrix for black (Bl) is next explained.It is assumed that the black (Bl) output is to have the screen angle of0 degrees. FIG. 20 shows a construction thereof. In FIG. 20, a 10×10square matrix is divided by four and resulting 5×5 square matrices areused as basic cells, because there is no need for forming the screenangle as the screen angle is 0 degrees. For the basic cell (5×5 squarematrix), the thresholds are determined in the same manner as that forthe threshold matrix of FIG. 11 (Fatting method), Because the screenangle is 0 degrees, a smaller 8×8 matrix may be used instead of the10×10 matrix. This is explained below. In FIG. 21, the black (Bl) isconstructed by an 8×8 threshold matrix. The 8×8 matrix has a shorter dotpitch of the grid points than the 10×10 matrix. As a result, theresolution power is improved. FIG. 21A shows the threshold matrix forthe ternary output and FIG. 21B shows the threshold matrix for thebinary output. For the black (Bl), the 8×8 threshold matrix is used and65 levels (levels 0-64) of black dots can be represented, The tonalityis lower than that of the cyan (C) or magenta (M) because the resolutionpower is emphasized. It is better for the black (Bl). As shown in FIG.19, the 8-bit input data (256 levels) is γ-converted to 65 levels. FIG.22 shows an example of black output dots. FIG. 22A shows an initialstage in which two basic cells are blackened, and in FIG. 22B, fourbasic cells are blackened.

As seen from FIG. 22, the dot pitch is constant and the dots of uniformdensity can be produced. Accordingly, the nonuniformity of density andthe disturbance of tonality are prevented.

The threshold matrix for yellow (Y) is next explained. The yellow (Y) isoutputted at the screen angle of 45 degrees. The yellow record is hardto be noticed and hard to be the subject of moire. Accordingly, noproblem arises although an angle difference from cyan (C) or magenta (M)is 18.4 degrees. The screen angle may even be 0 degrees in some cases.The thresholds of the basic cells for yellow (Y) are determined in thesame manner as that for the threshold matrix of FIG. 11 (Fattingmethod). FIGS. 23A and 23B show the basic cells for yellow (Y) in the8×8 matrix and the 10×10 matrix, respectively. For the yellow (Y), thetonality is important but the resolution is not so important.Accordingly, when the matrix is of 8×8 size, each of the basic cellscomprises 32 dots, and when the matrix is of 10×10 size, each of thebasic cells comprises 50 dots. The ternarization and the γ-conversioncan be done, as is done for other color. In the electrophotography, thetonality is hard to attain as the grid pitch is reduced.

Accordingly, it is desirable that the small basic cell which emphasizesthe resolution is used for black (Bl) and the large basic cell whichemphasizes the tonality is used for yellow (Y). The present invention isconstructed in this manner.

FIG. 24 illustrates superposition of dots of different colors havingscreen angles. For magenta (M) and cyan (C), the 10×10 matrices areused, and for black (Bl) the 8×8 matrix is used. The yellow (Y) dots arenot shown because of little influence. In FIG. 24, the screen angle formagenta (M) is 26.6 degrees, the screen angle for cyan (C) is 63.4degrees and the screen angle for black (Bl) is 0 degrees. The screenangle for yellow (Y) is shown by a broken line and it is 45 degrees. Byimparting different screen angles to the respective colors, an unnaturalstripe pattern is prevented.

FIG. 25 shows a reproduced image when different screen angles areimparted to the respective colors. Since the moire frequency is shiftedto a high frequency band, the unnatural stripe pattern does not appear.It was confirmed that, by setting the screen angles for the respectivecolors to those described above, the unnatural stripe pattern does notappear even if a paper is skewed and the screen angles for therespective colors are slightly varied. The present invention is notlimited to the above embodiment but also applicable to other colors (forexample, only black and gray).

Another embodiment of the present invention is explained below.

FIG. 26A shows a 12×12 threshold matrix which comprises 10 basic cells(cells 1-10). The basic cells of FIG. 26A are of special shape anddifferent from that of the basic cells described before. The shapes ofthe basic cells are different and the number of dots contained in thebasic cell is 14 or 15. This does not pose a problem in a recordedstate. The matrix has the screen angle of 18.4 degrees. When the matrixis used for cyan (C), for example, the matrix includes three types ofdot patterns (A), (B) and (C), as shown in FIG. 27.

FIGS. 28 and 29 show threshold matrices for producing the output dotpatterns (A) and (B) of FIG. 27. The threshold level in a blank area isa maximum level. FIGS. 28A and 29A show the threshold matrices for theternary output and FIGS. 28B and 29B show the threshold matrices for thebinary output. The threshold matrices of FIGS. 28A and 28B are allottedto the input data 0-7, and threshold matrices of FIGS. 29A and 29B areallotted to the input data 8-11. A threshold matrix (not shown) having acenter nuclear elements (an element which is first blackened in thebasic cell) in the output dot pattern of FIG. 27(C) and having a densitypattern thereof constructed by the Fatting method is allotted to theinput data of no smaller than 12. For magenta (M), the threshold matrixis rotated by 90 degrees as described above. For yellow (Y), the 12×12matrix as shown in FIG. 26B is used. In this case, the Fatting method isused, and by setting the center of each basic cell to a minimumthreshold level, the screen angle of 45 degrees is obtained. For black(Bl), the threshold matrix having the screen angle of 0 degrees is usedas described above. In the present embodiment, two threshold matricesare used for the density data of 0-11 in order to reproduce an image oflow density level (bright). According to the present embodiment, likethe previous embodiment, the dots are arranged in a uniform density anda high quality of image is reproduced. A hardware configuration of thepresent embodiment slightly differs from that of the previousembodiment. The counters 49 and 50 in FIG. 10 are modified to bidecimalcounters. For black (Bl), an octel or decimal counter is used.

Two comparators 42 are necessary in order to select one of threethreshold matrices for the input levels of 0-7, 8-11 and 12 or higher.Thus, two bits are used to select the threshold matrix. Two bits arenecessary for the input address to the ROM's 45a and 45b.

The ternarizing and binarizing circuits of the present invention havethus been described. By supplying the outputs of the binarizing andternarizing circuits directly to the output device 25 as shown in FIG.4, a high quality of color image sample is reproduced. When the speed ofthe input device and the output device are different or the outputtiming significantly shifts, four memories for yellow (Y), magenta (M),cyan (C) and black (Bl) are used instead of the output device 25. Whenthe dither method as shown in FIG. 1A is used, the 8 bits per picturecell information is stored in the memory as a dot pattern compressed toone bit by the processing circuit of the present invention. The outputof the memory is supplied to the color printer of FIG. 2.

The enlargement and the reduction of the image in accordance with thepresent invention are now explained.

In FIG. 4, the input device 20, the masking circuit 21, the blackeningcircuit 23, the binarizer/ternarizer circuit 24 and the output device 25are operated in synchronism with the picture cell clock 46. The outputdevice 25 supplies the horizontal synchronizing signal (BD signal) 48 tothe input device 20 and the binarizer/ternarizer circuit 24.Accordingly, the horizontal (H-direction) and vertical (V-direction)outputs of the image are switched in synchronism with the picture cellclock 46 and the horizontal synchronizing signal (BD signal) 48,respectively. By the arrangement of FIG. 4, the signal processingcircuit is simplified and it can be readily implemented by firmware.Accordingly, a sequence of signals can be processed at high speed onreal time basis. The one-to-N frequency divider 27 and the one-to-Mfrequency divider 28 are provided to frequency divide the frequencies ofthe picture cell clock 46 and the horizontal synchronizing signal 48 bythe factors of N and M, respectively.

Accordingly, the clock period is multiplied by N and the horizontalsynchronizing signal period is multiplied by M. The input device 20produces the picture cells of the input image in synchronism with the1/N picture cell clock 46 and the 1/M horizontal synchronizing signal48.

Assuming that N=M=4, the picture cell clock of the period multiplied byfour and the horizontal synchronizing signal of the period multiplied byfour are applied to the input device 20 so that the input device 20produces the image data at one-quarter speed.

Since the other circuits are operated at the normal speed, the outputdevice 25 produces 4×4 picture cells M data while the input device 20produces one picture cell of data, assuming that the input device 20reads the same line four times repetitively.

When N=M=1, the output device 25 produces one picture cell of data whilethe input device 20 produces one picture cell of data. Accordingly, thedither method shown in FIG. 1A is carried out.

Thus, for a given size of the picture cells produced by the input device20, an image horizontally magnified by a factor of N and verticallymagnified by a factor of M is produced. Since the dot pattern recordedis constant, the dots are not coarsened by the enlargement. As a result,a high quality of enlarged or reduced image can be readily produced.

In the present invention, the 8-bit input image data is used by way ofexample and the data may be determined depending on the characteristicsof the input device and the output device. While the output devicedescribed is of electrophotographic type which modulates thesemiconductor laser beam, an ink jet printer, a thermal transfer printeror an electrostatic printer also may be used. In this case, the ternarypulse width output is supplied in the sub-scan direction. The presentinvention is not limited to the constructions shown in FIGS. 4 and 10but any other signal processing circuit may be used as long as theprincipal means of the signal processing is maintained.

As described hereinabove, the present invention provides the imageprocessing apparatus which produces a high resolution and high tonalityimage output.

The present invention is not limited to the above embodiments butvarious modifications can be made within the scope defined in theappended claims.

What is claimed is:
 1. An image processing apparatus for producing animage from image data comprising:means for generating pixel data insequence; and means for processing the pixel data from said generatingmeans to produce a reproduction signal; wherein said processing meansincludes dither conversion means adapted to store data for ditherconversion, and means for determining the number of thresholds regardingone pixel data in accordance with a selected magnification, said datafor dither conversion in said dither conversion means corresponding to apredetermined threshold dither matrix composed of a plurality ofthresholds, wherein said determining means includes means for sending aclock signal having a predetermined period and means for changing thefrequency of the clock signal in accordance with the selectedmagnification, and wherein said dither conversion means performs ditherconversion of said pixel data in synchronism with the clock signalhaving the predetermined period; and wherein said generating meansgenerates said pixel data in synchronism with the frequency-changedclock signal output from said changing means.
 2. The image processingapparatus according to claim 1, wherein said clock signal is adapted tooutput the data stored in said dither conversion means;wherein saidchanging means includes means for dividing said clock signal, and thedivided clock signal is provided to said generating means for generatingthe image data.
 3. The image processing apparatus according to claim 2,wherein said dither conversion means includes means for storing athreshold signal corresponding to said predetermined threshold matrix,and said processing means includes a comparator for comparing the imagedata outputted in synchronism with said divided clock signal with thethreshold signal outputted in synchronism with said clock signal so asto form a binary signal.
 4. An image processing apparatus,comprising:image signal input means; variable magnification processingmeans for magnification-processing an image signal inputted by saidinput means; and halftone processing means for halftone-processing theimage signal magnification-processed by said variable magnificationprocessing means and providing the halftone-processed image signal to animage output device; wherein said halftone processing means processesthe magnification-processed image signal in synchronism with a clocksignal having a predetermined frequency regardless of the magnificationof said variable magnification processing means.
 5. An image processingapparatus according to claim 4, wherein said variable magnificationprocessing means includes oscillation means for generating the clocksignal and division means for dividing said clock in accordance withsaid variable magnification to generate a divided clock to be adapted asa transfer clock for transferring the image signal inputted by saidinput means.
 6. An image processing apparatus according to claim 5,wherein the clock signal is a sampling clock for sampling the imagesignal inputted in synchronism with the transfer clock.
 7. An imageprocessing apparatus according to claim 4, wherein said halftoneprocessing means includes dither processing means for dither-processingthe magnification-processed image signal.
 8. An image processingapparatus according to claim 7, wherein said dither processing meansincludes means for generating a threshold signal and means for comparingthe magnification-processed image signal with the threshold signal toproduce a binary signal.
 9. An image processing apparatus according toclaim 6, wherein said halftone processing means includes means forgenerating a threshold signal for halftone processing themagnification-processed image signal, the threshold signal beinggenerated in synchronism with the sampling clock.
 10. An imageprocessing apparatus, comprising:generating means for generating ascanned image signal in accordance with a magnification; and halftoneprocess means for halftone-processing the image signal generated by saidgenerating means, said halftone process means halftone-processing in ascanning direction, and for providing the halftone-processed imagesignal to an image output device; wherein said halftone processing meansprocesses the image signal generated by said generating means insynchronism with a signal having a predetermined frequency regardless ofthe magnification.
 11. An apparatus according to claim 10, wherein theimage output device records an image on a record medium on the basis ofthe image signal halftone-processed by said halftone process means. 12.An apparatus according to claim 10, wherein the image output devicecomprises a laser beam printer.
 13. An apparatus according to claim 10,wherein said generating means generates the image signal in accordancewith the magnification and the clock signal.
 14. An apparatus accordingto claim 10, wherein said generating means generates a color imagesignal composed of plural component signals, and said halftone processmeans halftone-processes each of the component signals.
 15. An apparatusaccording to claim 10, wherein said generating means comprises an imagereader.
 16. An apparatus according to claim 15, wherein said imagereader reads an image in response to a frequency divided synchronizingsignal from the image output device based on the magnification.
 17. Anapparatus according to claim 10, wherein said halftone process meanscomprises a threshold matrix, and performs halftone processing inaccordance with the threshold matrix.
 18. An image processing apparatus,comprising:generating means for generating pixel data for representingan image: memory means for storing a matrix composed of a plurality ofthresholds; means for permitting an allotment relation between imagedata and the thresholds in such a manner that one pixel data correspondsto the allotted thresholds based on a magnification; and producing meansfor producing a reproduction signal on the basis of a result bycomparison between the pixel data and the thresholds allotted by saidallotment means.
 19. An apparatus according to claim 18, wherein saidallotment means permits the allotment relation in such a manner that thenumber of thresholds which one pixel data corresponds to when enlargedis greater than that when equalsized.
 20. An apparatus according toclaim 18, wherein said allotment means controls said generating means togenerate pixel data in accordance with a magnification.
 21. An apparatusaccording to claim 20, wherein said producing means produces areproduction signal in accordance with a reference signal, and saidgeneration means generates pixel data in accordance with a magnificationand the reference clock signal.
 22. An apparatus according to claim 18,wherein said producing means produces a color reproduction signal. 23.An apparatus according to claim 18, wherein said generating meanscomprises an image reader.
 24. An apparatus according to claim 23,wherein said image reader reads an image in response to a frequencydivided reference clock signal based on a magnification.
 25. Anapparatus according to claim 4, wherein said image signal input meansinputs a color image signal.
 26. An apparatus according to claim 4,wherein said image signal input means comprises an image reader.
 27. Anapparatus according to claim 1, wherein said processing means produces acolor reproduction signal.
 28. An apparatus according to claim 1,wherein said generating means comprises an image reader.
 29. Anapparatus according to claim 4, wherein said halftone processing meansproduces a dot image according to a density level of the image signal.30. An apparatus according to claim 29, wherein said halftone processingmeans produces an image in which the number of dots in an areacorresponds to image density of the image signal in the area.
 31. Anapparatus according to claim 29, wherein said halftone processing meansproduces an image in which dot width corresponds to image density of theimage signal.
 32. An apparatus according to claim 4, wherein the imageoutput device records an image on the basis of the halftone-processedimage signal.
 33. An apparatus according to claim 4, wherein saidhalftone processing means performs a halftone process operation insynchronism with the synchronizing signal from the image output device.34. An apparatus according to claim 10, wherein said generating meansgenerates an image signal by photoelectrically converting an originalimage.
 35. An apparatus according to claim 10, wherein said halftoneprocess means performs a halftone process operation in synchronism withthe synchronizing signal from the image output device.
 36. An apparatusaccording to claim 35, wherein said generating means operates inaccordance with the clock signal depending upon the synchronizing signaland the variable magnification, and said halftone process means and saidimage output device operate in accordance with the clock signal that isindependent of the variable magnification.
 37. An apparatus according toclaim 10, wherein said halftone processing means produces a dot imageaccording to a density level of the image signal.
 38. An apparatusaccording to claim 37, wherein said halftone processing means producesan image in which the number of dots in an area corresponds to imagedensity of the image signal in the area.
 39. An apparatus according toclaim 37, wherein said halftone processing means produces an image inwhich dot width corresponds to image density of the image signal.
 40. Anapparatus according to claim 10, wherein said generating means includesa line sensor for reading an original image, and said line sensor scansthe original image at a sub-scan speed according to a variablemagnification.
 41. An image processing method comprising the stepsof:inputting an image signal; magnification-processing said image signalinputted by said inputting step; and halftone processing the imagesignal magnification-processed by said magnification processing step andproviding the halftone-processed image signal to an image output device,wherein in said halftone processing step, the image signal ishalftone-processed in synchronism with a clock signal having apredetermined frequency regardless of the magnification of saidmagnification processing step.
 42. An image processing method,comprising the steps of:generating an image signal in accordance with amagnification; halftone-processing the image signal generated by saidgenerating step; and providing the halftone-processed image signal to animage output device, wherein, in said halftone-processing step, theimage signal is halftone-processed in synchronism with a clock signalhaving a predetermined frequency regardless of the magnification.
 43. Anapparatus according to claim 4, wherein the synchronizing signalincludes a horizontal synchronizing signal.
 44. An apparatus accordingto claim 4, wherein said halftone processing means halftone processesthe image signal magnification processed in synchronism thesynchronizing signal.
 45. An apparatus according to claim 4, whereinsaid variable magnification processing means magnification-processes theimage signal in synchronism with a signal according to a magnificationand the synchronizing signal.
 46. An apparatus according to claim 4,wherein said variable magnification processing meansmagnification-processes the image signal in synchronism with a firstsignal depending upon a magnification, and said halftone processingmeans halftone-processes the image signal in synchronism with a secondsignal independent of the magnification.
 47. An apparatus according toclaim 4, wherein said image signal input means inputs an image signalfor a line in synchronism with the synchronizing signal.
 48. Anapparatus according to claim 10, wherein the image signal represents anoriginal image variable-magnified in accordance with the magnification.49. An apparatus according to claim 10, wherein said halftone processmeans halftone-processes the image signal generated in synchronism withthe synchronizing signal.
 50. An apparatus according to claim 10,wherein the synchronizing signal includes a horizontal synchronizingsignal.
 51. An apparatus according to claim 10, wherein said generatingmeans generates the image signal in synchronism with a signal accordingto the magnification and the synchronizing signal.
 52. An apparatusaccording to claim 10, wherein said generating means generates an imagesignal for a line in synchronism with the synchronizing signal.